We have recently reported the presence of Paul-Bunnell (PB) Antigen in the spleens of patients with Hodgkins, leukemia and oteer hematopoietic malignancies (K. Kano, A. Fjelde, F. Milgrom: J. Immun. 119:945, 1977) and at the present time are engaged in studies concerning this antigen, which is found in the cells of other species including sheep, cows, horses and mice, but not normally in man. The (PB) antibody to this antigen appears in man during the acute stage of infectious mononucleosis (IM) and subsequently disappears. Although patients with Hodgkins, leukemia and other hematopoietic diseases have the PB antigen, they do not apparently, like IM patients, produce PB antibodies. Using IM sera, we have detected and reported the presence of PB antigen, but have never identified in histological preparations or cell cultures the cell types associated with the presence of the antigen. We propose to employ autoradiography, using 125I-conjugated antibody from high titered IM convalescent sera in these studies. We are especially interested in Hodgkin's disease. The infliltrates of Hodgkin's disease are generally composed of a number of different kinds of cells, a characteristic cell being the Reed-Sternberg cell. Which of the cells are associated with the PB antigen (or the origin of the antigen) is at present not known. The specific aim of the proposed work is to identify the cell or cells, using immuno-histological techniques.